This invention relates to a free-flowing, homogeneous polymer-coated pigment, particulate powder coating composition which is adapted for application by an electrostatic spraying process and to the method for its preparation.
While the invention taught herein provides a free-flowing, homogeneous, polymer-coated pigment, particulate powder coating composition and herein is referred to as such, and, as already mentioned, is useful in electrostatic spraying processes for coating purposes, it is to be understood that this composition has other and additional uses. For example, this invention's composition also is useful in printing ink compositions and the like appliable by electrostatic means, and additionally this invention's composition is useful as and in toner compositions used in xerographic copying machines and the like.
It is known that compositions for coating purposes include acrylic polymers and that such polymers can be formed by polymerizing an acrylic monomer in a non-aqueous dispersion in an aliphatic hydrocarbon liquid using a dispersion stabilizer. The general principles of making non-aqueous dispersions of polymer in organic liquids by polymerizing a monomer in an organic liquid in which the resulting polymer is insoluble, using a dispersion stabilizer to stabilize the resulting particles of insoluble polymer in the organic liquid, are known and described in the literature. A review of the published literature including patents relating to non-aqueous dispersions and dispersion stabilizers appears in "Nonaqueous Dispersions as Vehicles for Polymer Coatings" by R. Dowbenko and D. P. Hart, Ind. Eng. Chem Prod. Res. Develop. Vol. 12, No. 1, 1973. Non-aqueous dispersions, procedures by which they are made and uses for such dispersions are set forth in U.S. Pat. Nos. 3,095,388; 3,317,635; 3,607,821; 3,666,710; and 3,686,111. British Pat. Nos. 941,305 and 1,052,241; and French Pat. No. 1,478,438. The disclosures in these patents as well as the disclosure in the review article by R. Dowbenko and D. P. Hart referred to above, provide information for selection of appropriate monomers, catalysts, stabilizers, polymerization conditions, etc. useful in practicing the present invention.
A dispersion stabilizer comprises a polymeric component which is solvatable by the aliphatic hydrocarbon and a component which is relatively non-solvatable by the aliphatic hydrocarbon and is associated with the dispersed polymer particles. In the invention, the employed monomer and/or monomers are soluble in the aliphatic hydrocarbon, while the polymer formed in insoluble in the aliphatic hydrocarbon and is provided as a polymeric dispersion. The polymeric dispersion per se, may then be applied to the object to be coated. It has been preferred however to dilute the dispersion with an organic solvent thinner, and a coalescing solvent and then spray the diluted polymeric dispersion onto the object to be coated. A coating of this type is disclosed in U.S. Pat. No. 3,666,710. In this patent and also as generally understood in the art and for purposes of the present invention, a coalescing solvent is an active solvent for the polymer of concern. The use of an organic solvent thinner has certain disadvantages in that it increases the cost of the final product, reduces the coverage in any single pass, creates an environmental pollution problem upon its evaporation and may give rise to runs or sags if applied to freely. The use of a coalescing solvent is desirable in producing liquid coating compositions but a coalescing solvent is undesirable in producing powdered coatings since coalescing solvents become concentrated in the polymerizate upon evaporation of the other diluents. As the concentration of coalescing solvent increases in the polymerizate, there is a tendency for the polymer particles to become fused during the end of the drying process. If, on the other hand, a coalescing solvent is not used in preparing stabilizers such as those disclosed in U.S. Pat. Nos. 3,607,821; 3,666,710; and 3,686,111; gelation of the stabilizers occurs during their synthesis which renders then unsuitable for subsequent use in preparing a free-flowing, pigmented, powdered coating composition.
Powdered coating compositions have been prepared by dry blending a thermosetting film-forming resin such as a phenolic resin with a solid infusible material such as a silicone resin. Such powdered coatings are disclosed in U.S. Pat. No. 3,645,960. Blending procedures, however, are time-consuming and may result in heterogeneous mixtures. In addition, if pigmented coatings are desired, the blend may not result in a uniformly-colored coating.
Stable dispersions in an organic liquid of solid pigment particles encapsulated by a synthetic polymer and coating compositions based on such dispersions are disclosed in Irish Pat. No. 66,933P. According to the Irish patent, the pigment particles are given a preparatary treatment by first dispersing them in an organic liquid containing both a polymer to be adsorbed on the pigment and a dispersion stabilizer. The dispersion of pigment particles having polymer adsorbed thereon is then admixed with other monomer components and additional dispersion stabilizer. The admixture is then subjected to non-aqueous dispersion polymerization whereupon there is obtained a dispersion of a polymer-encapsulated pigment in an organic liquid. The dispersion in the organic liquid can then be used as prepared or it can be used to pigment other similar or dissimilar non-aqueous dispersion polymers. While such liquid dispersions of polymer-coated pigment can be used, per se, as coating compositions, there remains the problem of polluting the atmosphere with the organic liquid upon its evaporation to produce a dry coating. If, on the hand, the organic liquid is removed in an attempt to form a dry powder without prior removal of any coalescing solvent, such as, for example, n-butyl acetate or toluene, there is a tendency for the particles to agglomerate and form globules unsuitable for application by spraying techniques, such as electrostatic spraying.